FIELD OF THE INVENTION
The invention relates to a burner for gas cookers fitted with three-flame crowns, in particular a burner suitable for use in built-in gas-hobs.
Burners for gas cookers with two or three-flame crowns are known in the present state of the art. In particular, three-crown burners are known of which one is central and two more are external crowns; of the latter, one is turned inwards and the other towards the outside of the burner.
These burners allow uniform heating of the overlying pots irrespective of the shape of the pot base.
In the application for U.K. Patent No. GB-A-2233444, in the name of Lee, a burner is described comprising a vertical duct, which both supports the burner and feeds the gas, on which was fitted a three-crown head. The central crown is coaxial with the vertical duct and communicates with it; three connecting arms branch off from the duct to feed the mixture of primary air and gas to an external circular flame separator ring, on which are drilled the two circumferential crowns for external and internal flames.
This type of embodiment, however, is not suitable for use in gas hobs, where the low profile of the burners does not permit the use of long ducts for mixing the gas with the air. Furthermore, the internally directed circumferential flame crown has a reduced number of flames because of the interference between the facing crowns and the consequent difficulty of combustion for this type of burner.
The German Patent No. 3123751, in the name of the applicant for the present invention, describes a burner where both the primary and secondary air is drawn from above the hob and the mixture of primary air and gas is supplied to the flame-crowns through an annular chamber by of Venturi expansion effect, the horizontal Venturi.
The European Patent Application No. 0534301 in the name of Merloni, described a burner for gas cookers with a double flame-crown head. The two crowns are drilled externally and internally in a flame-separator ring which is divided internally by septal separators into sectors into which the primary air-gas mixture is supplied through a corresponding manifold. The latter embodiment, while on one hand resolving the problem of the low height profile of the burner with the adoption of the horizontal Venturi, has not resolved the problems of the prior art in that the burner is fitted with only two circumferential flame-crowns of which the more internal presents a number of extremely reduced flames and therefore insufficient distribution of the thermal energy toward the center of the burner.
EP-A-96109934.8 (Art. 54(3) EPC), in the name of the present applicant, describes a three-flame crown burner fitted with a horizontal venturi produced in a chamber formed between the base of the burner and an upper portion comprising the flame-crowns. The Venturi chamber communicates with an upper chamber defined by the base of the zone of the central crown and the central flame separator cover. This solution gives excellent performance but it has been found that it is often necessary to provide for the annealing of the burner after having bound the base of the burner to its upper portion, to eliminate the tensions generated.